The field of art to which this invention pertains is aqueous epoxy resin/acrylic acid copolymer dispersions useful as can coating compositions.
In the manufacture of metal containers, a thin protective synthetic resin coating is applied to the interior of the container to prevent contact of the metal surface of the can with its contents. Such coatings ideally should have good adhesion to the interior metal surface, low extractables to prevent contamination of the container contents, and a rapid cure or drying rate for economy of container manufacture. Synthetic resin coating compositions which have been used for a number of years include organic solutions of vinyl resins, polybutadiene resins, epoxy resins, aminoplast and phenolplast resins and oleoresinous materials.
In order to reduce the use of organic solvents, efforts have been made to convert the coating compositions to water-borne systems. One such system is described in U.S. Pat. No. 4,247,439 wherein a blend of an epoxy resin, an acrylic copolymer which contains carboxylic acid groups and a tertiary amine are dispersed in water.
Another approach to an aqueous coating composition is described in U.S. Pat. No. 4,289,811. Acrylic or methacrylic acid is copolymerized with other monomers in the presence of an aminoplast resin followed by the addition of an epoxy resin and an amine. The resulting blend is then dispersed in water.
In U.S. Pat. No. 4,212,781, acrylic or methacrylic acid is copolymerized with another monomer in the presence of an epoxy resin, a glycidyl polyether of a dihydric phenol. Some of the resulting copolymer forms a graft polymer with the backbone of the epoxy resin, graft polymer and acrylic acid copolymer, which is then neutralized with an amine and dispersed in water.
U.S. Pat. No. 4,442,246 describes sanitary can coating compositions made from the reaction product of an acid copolymer and a partially terminated epoxy resin, neutralized with an amine and dispersed in water.
In U.K. Pat. No. 2,068,967, epoxy resins are partially reacted with acrylic acid copolymers and the reaction product is neutralized with an amine and dispersed in water to form coating compositions.
Water-borne epoxy resin coting compositions, such as those described above, have replaced organic solvent-borne coatings in many can coating applications. However, none of these coatings has been found to be adequate replacements for modified polyvinyl chloride organosol coatings for use as "non-repair" coatings for can ends. Can ends, such as "easy open" beer and beverage can ends, are fabricated from pre-coated metal. The coating must have good flexibility and toughness in order to withstand the rigors of fabrication such as forming and seaming.